Liquid crystal display

ABSTRACT

The present invention relates to a liquid crystal display, comprising a lower alignment film formed on a lower substrate; an upper alignment film formed on an upper substrate; a liquid crystal layer sandwiched between the lower and upper substrates; a phase compensation film adhered on the outer surface of the upper substrate; and a polarizer adhered on the phase compensation film wherein the lower alignment film has an alignment angle of −10 to 20° with respect to a horizontal line, the upper alignment film has an alignment angle of 40 to 55° with respect to a horizontal line, the liquid crystal layer has a phase delay value (dΔn) of 0.24-0.27 μm, the phase compensation film has a phase compensation function of λ/4 and also has an optical axis making 140-146° with a horizontal line, and the polarizer has a absorption axis making 120 to 122.5° with a horizontal line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a liquid crystal display, and moreparticularly, to a liquid crystal display device having optimizedcharacteristic parameters, and thus having improved displaycharacteristics and increased cell gap margin.

2. Description of the Prior Art

Generally, liquid crystal displays are divided into a transmissiveliquid crystal display using backlight as a light source, a reflectiveliquid crystal display using natural light without backlight, and atransflective liquid crystal display, which selectively uses backlightor natural light according to circumstance.

Among such display devices, the reflective liquid crystal displayrequiring no backlight is useful for portable display systems, since itcan have low consumption power, thin thickness and lightweight. As themarket of mobile phones and portable devices is extended, a demand forthe reflective liquid crystal display is gradually increased.

This reflective liquid crystal display comprises a lower substrate, areflective electrode, a lower alignment film, a liquid crystal layer, anupper alignment film, a transparent electrode, a color filter, an uppersubstrate, a phase compensation film, and a polarizer, which aresuccessively stacked on top of each other.

Moreover, the liquid crystal displays can be divided into TN (Twistednematic) mode, GH (Guest Host) mode, ECB (Electrically ControlledBirefringence) mode, and OCB (Optically Compensated Birefringence) mode,according to their operation mode.

The TN liquid crystal display is currently widely used in computers andmeasurement devices, but has the problem of slow response speed. The ECBliquid crystal display is a mode of inducing a change in lighttransmittance by a change in birefringence of liquid crystal cells, andits typical example includes a HAN (Hybrid-Aligned Nematic) mode. TheHAN mode liquid crystal display is actively studied since it hasadvantages in that it is operated at relatively low electric power andhas fast operation speed.

Hereinafter, the structure and display characteristics of the reflectiveliquid crystal display will be briefly described.

FIG. 1 is a cross-sectional view, which schematically shows a reflectiveliquid crystal display according to the prior art. As shown in FIG. 1,the reflective liquid crystal display according to the prior artcomprises a lower substrate 1 comprising a reflective electrode 2 and alower alignment film 3, and an upper substrate 4 comprising a colorsubstrate 2 and a upper alignment film 6 and disposed opposite to thelower substrate 1 while sandwiching a liquid crystal layer 10therebetween. And on the outer surface of the upper substrate 4, a phasecompensation film 7, such as a λ/4 film, and a polarizer 8, aresuccessively adhered. The reflective electrode 2 has a rugged portion onits surface, which is formed by a lithography or holography process.

In this reflective liquid crystal display, upon no voltage application,light which was linearly polarized by passage through the polarizer isconverted into circularly polarized light, such asleft-circularly-polarized light, by passage through the phasecompensation film. The circularly polarized light is converted intolinearly polarized light by passage through the liquid crystal layer andreflected in the reflective electrode. Then, the linearly polarizedlight reflected in the reflective electrode is converted intoleft-circularly-polarized light by passage through the liquid crystallayer, after which it is passed through the phase compensation film andthus converted into linearly polarized light whose polarizing directionis parallel to the polarizing axis of the polarizer. Then, the linearlypolarized light is passed through the polarizer to achieve a whitestate.

Moreover, in the reflective liquid crystal display, upon voltageapplication, light which was converted into left-circularly-polarizedlight by passage through the phase compensation film is passed throughthe liquid crystal layer intact and reflected in the reflectiveelectrode to convert it into right-circularly-polarized light. Thisright-circularly-polarized light is passed through the liquid crystallayer and the phase compensation film to convert it into linearlypolarized light whose polarizing direction is perpendicular to thepolarization axis of the polarizer. Thus, the linearly polarized lightcannot be passed through the polarizer so that a dark state is achieved.

In the reflective liquid crystal display, good display characteristicsvary on how to optimize a characteristic value of the respectiveelements as described above. In other words, for an efficient increasein reflectance in the reflective liquid crystal display, the angle ofthe absorption axis or transmission axis of the polarizer, the opticalcharacteristics of the phase compensation film, the thickness (d) of theliquid crystal layer, the phase delay value (dΔn) of the liquid crystallayer, the twist angle of liquid crystal molecules, the alignment angleof the alignment film, and the characteristics of the reflective film,etc., must be optimized.

For example, in the prior reflective liquid crystal display, gooddisplay characteristics can be obtained when one or two λ/4 films 7 areused, the alignment angle of the upper alignment film 6 is perpendicularto that of the lower alignment film, and the transmission axis of thepolarizer 8 makes a 20° degree with the alignment angle of the upperalignment film 6 while making a 45° angle with the optical axis angle ofthe λ/4 film 7. In this case, the twist angle of liquid crystals and thephase delay value of the liquid crystal layer are controlled to about 63to 80°, and 0.20 to 0.27 μm, respectively.

However, in the prior reflective liquid crystal display, if two λ/4films are used for the optimization of a cell design, a reduction inreflectivity will be caused, whereas if one λ/4 film is used, a functionof providing a phase difference of λ/4 over a wide range of visiblelight wavelengths will not sufficiently performed so that bad displaycharacteristics will be caused.

In addition, in the prior reflective liquid crystal display, cell gapprovided for obtaining good display characteristics is extremely small,and thus, yield in an actual process for the production of the priorreflective crystal display is disadvantageously reduced.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made to solve theabove-mentioned problems occurring in the prior art, and an object ofthe present invention is to provide a liquid crystal display havingimproved display characteristics.

Another object of the present invention is to provide a liquid crystaldisplay having ensured sufficient cell gap and thus improved processmargin.

To achieve the above objects, in one embodiment, the present inventionprovides a liquid crystal display, which comprises: a lower substratehaving a reflective electrode; a lower alignment film formed on thelower substrate; an upper substrate having a color filter and disposedopposite to the lower substrate; an upper alignment film formed on theupper substrate; a liquid crystal layer sandwiched between the lower andupper substrates; a phase compensation film adhered on the outer surfaceof the upper substrate and serving to convert linearly polarized lightinto circularly polarized light; and a polarizer adhered on the phasecompensation film and serving to convert natural light from the externalinto linearly polarized light, wherein the lower alignment film has analignment angle of −10 to 20° with respect to a horizontal line; theupper alignment film has an alignment angle of 40 to 55° with respect toa horizontal line; the liquid crystal layer has a phase delay value(dΔn) of 0.24-0.27 μm; the phase compensation film has a phasecompensation function of λ/4 and also has an optical axis making anangle of 140 to 146° with a horizontal line; and the polarizer has aabsorption axis making an angle of 120 to 122.5° with a horizontal line.

In the liquid crystal display according to this embodiment, the liquidcrystal layer is preferably made of twisted nematic liquid crystalshaving a twist angle of 60°.

In another embodiment, the present invention provides a liquid crystaldisplay, which comprises: a lower substrate having a reflectiveelectrode; a lower alignment film formed on the lower substrate; anupper substrate having a color filter and disposed opposite to the lowersubstrate; an upper alignment film formed on the upper substrate; aliquid crystal layer sandwiched between the lower and upper substrates;a phase compensation film adhered on the outer surface of the uppersubstrate and serving to convert linearly polarized light intocircularly polarized light; and a polarizer adhered on the phasecompensation film and serving to convert natural light from the externalinto linearly polarized light; wherein the lower alignment film has analignment angle of 0 to 10° with respect to a horizontal line; the upperalignment film has an alignment angle of 55 to 65° with respect to ahorizontal line; the liquid crystal layer has a phase delay value (dΔn)of 0.23-0.27 μm; the phase compensation film has a phase compensationfunction of λ/4 and also has an optical axis making an angle of 160 to168° with a horizontal line; and the polarizer has a absorption axismaking an angle of 117.5 to 127.5° with a horizontal line.

In the liquid crystal display according to this embodiment, the liquidcrystal layer is preferably made of twisted nematic liquid crystalshaving a twist angle of 76°.

In still another embodiment, the present invention provides a liquidcrystal display which comprises: a lower substrate having a reflectiveelectrode; a lower alignment film formed on the lower substrate; anupper substrate having a color filter and disposed opposite to the lowersubstrate; an upper alignment film formed on the upper substrate; aliquid crystal layer sanwitched between the lower and upper substrates;a phase compensation film adhered on the outer surface of the uppersubstrate and serving to convert linearly polarized light intocircularly polarized light; and a polarizer adhered on the phasecompensation film and serving to convert natural light from the externalinto linearly polarized light; wherein the lower alignment film has analignment angle of 0 to 10° with respect to a horizontal line; the upperalignment film has an alignment angle of 50 to 56° with respect to ahorizontal line; the liquid crystal layer has a phase delay value (dΔn)of 0.17-0.21 μm; the phase compensation film has a phase compensationfunction of λ/4 and also has an optical axis making an angle of 135 to145° with a horizontal line; and the polarizer has a absorption axismaking an angle of 62-66° with a horizontal line.

In the liquid crystal display according to this embodiment, the liquidcrystal layer is made of twisted nematic liquid crystals having a twistangle of 53°.

In yet another embodiment, the present invention provides a liquidcrystal display, which comprises: a lower substrate having a reflectiveelectrode; a lower alignment film formed on the lower substrate; anupper substrate having a color filter and disposed opposite to the lowersubstrate; an upper alignment film formed on the upper substrate; aliquid crystal layer sandwiched between the lower and upper substrates;a phase compensation film adhered on the outer surface of the uppersubstrate and serving to convert linearly polarized light intocircularly polarized light; and a polarizer adhered on the phasecompensation film and serving to convert natural light from the externalinto linearly polarized light; wherein the lower alignment film has apre-tilt angle selected from 80 to 90° and less than 2°, and analignment angle of −40 to −50° with respect to a horizontal line; theupper alignment film has a pre-tilt angle selected from 2° or less and80 to 90° in the opposite manner to the lower alignment film, and analignment angle of 40 to 50° with respect to a horizontal line; theliquid crystal layer is a HAN-mode liquid crystal layer serving toconvert the phase of transmitted light depending on electric fieldapplication or no electric field application and has a phase delay value(dΔn) of 0.36-0.40 μm; the phase compensation film has a phasecompensation function of λ/4 and also has an optical axis making anangle of 167 to 173° or 7 to 13° with a horizontal line; and thepolarizer has a transmission axis making an angle of 42 to 48° with ahorizontal line.

In the liquid crystal display according to this embodiment, the liquidcrystal layer is preferably made of twisted nematic liquid crystalshaving a twist angle of 90°. The phase compensation film preferably hasa phase compensation delay value of 0.14-0.15 μm.

In still another embodiment, the present invention provides a liquidcrystal display which comprises: a lower substrate having a reflectiveelectrode; a lower alignment film formed on the lower substrate; anupper substrate having a color filter and disposed opposite to the lowersubstrate; an upper alignment film formed on the upper substrate; aliquid crystal layer sandwiched between the lower and upper substrates;a phase compensation film adhered on the outer surface of the uppersubstrate and serving to convert linearly polarized light intocircularly polarized light; and a polarizer adhered on the phasecompensation film and serving to convert natural light from the externalinto linearly polarized light; wherein a unit pixel of the liquidcrystal display is divided into first and second regions; a portion ofthe lower alignment film corresponding to the first region is treatedsuch that it vertically or horizontally aligns liquid crystal moleculesin the first region, and a portion of the lower alignment regioncorresponding to the second region is treated such that it horizontallyor vertically aligns liquid crystal molecules in the second region inthe opposite manner to the first region, the lower alignment film havingan alignment angle of −40 to −50° with respect to a horizontal line; aportion of the upper alignment film corresponding to the first region istreated such that it vertically or horizontally aligns liquid crystalmolecules in the first region in the opposite manner to the portion ofthe lower alignment film corresponding to the first region, and aportion of the upper alignment region corresponding to the second regionis treated such that it horizontally or vertically aligns liquid crystalmolecules in the second region in the opposite manner to the firstregion, the upper alignment film having an alignment angle of 40 to 50°with respect to a horizontal line; the liquid crystal layer is aHAN-mode liquid crystal layer serving to convert the phase oftransmitted light depending on electric field application or no electricfield application and has a phase delay value (dΔn) of 0.36-0.40 μm; thephase compensation film has a phase compensation function of λ/4 andalso has an optical axis making an angle of 167 to 173° with ahorizontal line; and the polarizer has a transmission axis making anangle of 42 to 48° with a horizontal line.

In the liquid crystal display according to this embodiment, the loweralignment film and the upper alignment film preferably have differentpre-tilt angles of 2° or less and 80 to 90°. The liquid crystal layer ispreferably made of twisted nematic liquid crystals having a twist angleof 90°. The phase compensation film preferably has a phase compensationdelay value of 0.14-0.15 μm.

In yet another embodiment, the present invention provides a liquidcrystal display which comprises: a lower substrate having a reflectiveelectrode; a lower alignment film formed on the lower substrate; anupper substrate having a color filter and disposed opposite to the lowersubstrate; an upper alignment film formed on the upper substrate; aliquid crystal layer sandwiched between the lower and upper substrates;a phase compensation film adhered on the outer surface of the uppersubstrate and serving to convert linearly polarized light intocircularly polarized light; and a polarizer adhered on the phasecompensation film and serving to convert incident light into linearlypolarized light; wherein a unit pixel of the liquid crystal display isdivided into a transmission region and a reflection region having thereflective electrode; a portion of the lower alignment filmcorresponding to the reflection region is treated such that itvertically or horizontally aligns liquid crystal molecules in thereflection region, and a portion of the lower alignment regioncorresponding to the transmission region is treated such that ithorizontally or vertically aligns liquid crystal molecules in thetransmission region in the opposite manner to the reflection region, thelower alignment film having an alignment angle of −40 to −50° withrespect to a horizontal line; a portion of the upper alignment filmcorresponding to the reflection region is treated such that itvertically or horizontally aligns liquid crystal molecules in thereflection region in the opposite manner to the portion of the loweralignment film corresponding to the reflection region, and a portion ofthe upper alignment region corresponding to the transmission region istreated such that it horizontally or vertically aligns liquid crystalmolecules in the transmission region in the opposite manner to thereflection region, the upper alignment film having an alignment angle of40 to 50° with respect to a horizontal line; a portion of the liquidcrystal layer corresponding to the reflection region is made of aHAN-mode liquid crystal layer, and a portion of the liquid crystalregion corresponding to the transmission region is made of a TN modeliquid crystal layer; the phase compensation film has a phasecompensation function of λ/4 and also has an optical axis making anangle of 167 to 173° with a horizontal line; and the polarizer has atransmission axis making an angle of 42 to 48° with a horizontal line.

In the liquid crystal display according to this embodiment, the loweralignment film and the upper alignment film preferably have differentpre-tilt angles of 2° or less and 80 to 90°. The HAN mode liquid crystallayer and the TN mode liquid crystal layer are preferably made oftwisted nematic liquid crystals having a twist angle of 90°. The HANmode and TN mode liquid crystal layers preferably have a phase delayvalue (dΔn) of 0.36-0.40 μm. The phase compensation film preferably hasa phase compensation delay value of 0.14-0.15 μm.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will be more apparent from the following detailed descriptiontaken in conjunction with the accompanying drawings, in which:

FIG. 1 is a cross-sectional view showing a reflective liquid crystaldisplay according to the prior art;

FIGS. 2 to 4 show the axial arrangement of elements of liquid crystaldisplays according to an embodiment of the present invention;

FIGS. 5 to 7 are graphs showing a change in reflectance according tovoltage of liquid crystal displays according to an embodiment of thepresent invention;

FIG. 8 is a graph showing a change in reflectance according to cell gap;

FIGS. 9 and 11 show the axial arrangement of elements of liquid crystaldisplays according to other embodiments of the present invention;

FIGS. 10 and 12 show a change in reflectance according to voltage inliquid crystal displays according to other embodiments of the presentinvention;

FIG. 13 is a cross-sectional view showing a multi-domain twisted HANliquid crystal display according to the present invention; and

FIG. 14 is a cross-sectional view showing a transflective liquid crystaldisplay having the combined structure of a twisted HAN mode and a TNmode.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A liquid crystal display of the present invention comprises a lowersubstrate including a lower alignment film and a reflective electrodehaving a rugged portion on its surface; an upper substrate including acolor filter and an upper alignment film; a liquid crystal layersandwiched between the upper and lower substrates; a phase compensationfilm adhered on the outer surface of the upper substrate and serving toconvert linearly polarized light into circularly polarized light; and apolarizer adhered on the phase compensation film and serving to convertnatural light from the external into linearly polarized light.

In the liquid crystal display of the present invention, the lower andupper substrates have the same thickness and refractive index of about0.4-0.7t and 1.5, respectively. Also, the thickness and refractive indexof a counter electrode in the upper substrate are about 1,500 Å and 1.6,respectively, and the thickness and refractive index of the upperalignment film are about 750 Å and 1.6. Moreover, the reflectiveelectrode made of AlNd has a thickness of about 1,500 Å.

In order to more effectively align liquid crystal molecules, the upperand lower alignment films are made of polyamic acid-based materials inview of hydropilicity with the liquid crystal molecules and adhesion tothe substrates.

The phase compensation film is formed of one λ/4 film. The λ/4 film isproduced by uniaxial orientation of a polymer film, such aspolycarbonate, polyvinyl alcohol, or polystyrene film, such that it hasa certain phase difference.

Furthermore, the liquid crystal display of the present invention has thecell structure optimized by the control of characteristic values of itselements, and thus, shows good display with high contrast ratio, goodcolor characteristics and high reflectance, and also has increased cellgap margin.

Parameters to be considered in making the cell structure optimal includethe angle of the absorption axis or transmission axis of the polarizer,the angle of the optical axis of two birefringent films forming the λ/4film, the phase delay value of the liquid crystal layer, and thealignment angle of the alignment film, etc.

Embodiment 1

The characteristic values of elements according to a first embodiment ofthe present invention, which can make the cell structure optimal, are asfollows.

Embodiment 1-1

A liquid crystal layer in a liquid crystal display according to thisembodiment is made of TN liquid crystals having a twist angle of 60°. Ifthe liquid crystal layer has a phase delay value of 0.24-0.27 μm, aliquid crystal display having increased cell gap margin, high contrastratio and excellent color characteristics will be obtained.

In this case, the alignment angle of the lower alignment film which canmake a cell structure optimal is −10 to −20° with respect to ahorizontal line, the alignment angle of the upper alignment film is 40to 55° with respect to a horizontal line, the optical axis of the λ/4film makes an angle of 140 to 146° with a horizontal line, and theabsorption axis of the polarizer makes an angle of 102 to 122.5° with ahorizontal line.

FIG. 2 shows the axial arrangement of elements of the liquid crystaldisplay according to this embodiment. In FIG. 2, the reference numeral adesignates the rubbing axis of the lower substrate, i.e., the alignmentangle of the lower alignment film, b the rubbing axis of the uppersubstrate, i.e., the alignment angle of the upper alignment film, c theoptical axis of the λ/4 film, and d the absorption axis of thepolarizer.

Embodiment 1-2

A liquid crystal layer in a liquid crystal display according to thisembodiment is made of TN liquid crystals having a twist angle of 76°. Ifthe liquid crystal layer has a phase delay value of 0.23-0.27 μm, aliquid crystal display having increased cell gap margin, high contrastratio and excellent color characteristics will be obtained.

In this case, the alignment angle of the lower alignment film which canmake a cell structure optimal is 0 to 10° with respect to a horizontalline, the alignment angle of the upper alignment film is 55 to 65° withrespect to a horizontal line, the optical axis of the λ/4 film makes anangle of 160 to 168° with a horizontal line, and the absorption axis ofthe polarizer makes an angle of 117 to 127.5° with a horizontal line.

FIG. 3 shows the axial arrangement of elements of the liquid crystaldisplay according to this embodiment. In FIG. 3, the reference numeral adesignates the rubbing axis of the lower substrate, i.e., the alignmentangle of the lower alignment film, b the rubbing axis of the uppersubstrate, i.e., the alignment angle of the upper alignment film, c theoptical axis of the λ/4 film, and d the absorption axis of thepolarizer.

Embodiment 1-3

A liquid crystal layer in a liquid crystal display according to thisembodiment is made of TN liquid crystals having a twist angle of 53°. Ifthe liquid crystal display has a phase delay value of 0.17-0.21 μm, aliquid crystal display having increased cell gap margin, high contrastratio and excellent color characteristics will be obtained.

In this case, the alignment angle of the lower alignment film which canmake a cell structure optimal is 0 to 10° with respect to a horizontalline, the alignment angle of the upper alignment film is 50 to 56° withrespect to a horizontal line, the optical axis of the λ/4 film makes anangle of 135 to 145° with a horizontal line, and the absorption axis ofthe polarizer makes an angle of 62 to 66° with a horizontal line.

FIG. 4 shows the axial arrangement of elements of the liquid crystaldisplay according to this embodiment. In FIG. 4, the reference numeral adesignates the rubbing axis of the lower substrate, i.e., the alignmentangle of the lower alignment film, b the rubbing axis of the uppersubstrate, i.e., the alignment angle of the upper alignment film, c theoptical axis of the λ/4 film, and d the absorption axis of thepolarizer.

In the embodiments as described above, when the twist angle of liquidcrystals is 60°, the angle between the alignment angle of the upperalignment film and the transmission angle of the polarizer is designedas 28 to 32°, and when the twist angle of the liquid crystals is 76°,the angle between the alignment angle of the upper alignment film andthe transmission angle of the polarizer is designed as 31.5 to 37.5°.

As described above, according to the present invention, the twist angleof liquid crystals is designed as 50°, 63° and 76° while the phase delayvalue of the liquid crystal layer is designed as 0.24-0.27 μm, 0.23-0.27μm and 0.17-0.21 μm, respectively. In addition, the alignment angle ofthe alignment films, the optical axis angle of the λ/4 film, and theabsorption or transmission axis angle of the polarizer, are suitablycontrolled. For this reason, the present invention allows the productionof a liquid crystal display having good display characteristics.Furthermore, the liquid crystal display of the present invention willhave a cell gap of about 3.6-4.0 μm, and thus, increased cell gap marginas compared to that of the prior liquid crystal display having a cellgap of about 2.9-3.5 μm.

FIGS. 5 to 7 are graphs showing a change in reflectance according tovoltage in liquid crystal displays. FIG. 5 is a graph showing a changein reflectance according to voltage in a reflective liquid crystaldisplay manufactured by Matsushita Co., Ltd., Japan. FIGS. 6 and 7 aregraphs showing a change in reflectance according to voltage in inventivereflective liquid crystal displays having twist angles of liquid crystalmolecules of 60° and 70°, respectively.

As shown in FIG. 5, the Matsushita's reflective liquid crystal displaycannot achieve a complete dark state upon voltage application. On theother hand, as shown in FIGS. 6 and 7, the reflective liquid crystaldisplays according to the present invention achieve a dark state uponvoltage application. Thus, it can be found that the liquid crystaldisplays of the present invention have a superior reflectancecharacteristic according to voltage than that of the Matsushita'sreflective liquid crystal display.

Furthermore, in the liquid crystal display according to the presentinvention, drive voltage can be reduced while maintaining contrast ratioat the same level.

As a result, the liquid crystal display according to the firstembodiment of the present invention is designed to have the twist angleof liquid crystal molecules of 76°, the phase delay value of the liquidcrystal layer of 0.25 μm, and the cell gap of the liquid crystal layerof 3.6 μm, so that it can have good display characteristics and can beeasily applied in an actual process due to an increase in cell gap.

Embodiment 2

The characteristic values of elements according to a second embodimentof the present invention, which can make the cell structure optimal, areas follows.

A liquid crystal layer in a liquid crystal display according to thisembodiment is made of an HAN mode liquid crystal layer, which convertsthe phase of transmitted light depending on whether electric field isapplied or not. This HAN mode liquid crystal layer is made of twistednematic liquid crystal molecules having a twist angle of 90°. The HANmode liquid crystal layer has a phase delay value (dΔn) of 0.36-0.40 μm,the transmission axis of the polarizer makes an angle of 42 to 48° witha horizontal line, and the optical axis of the λ/4 film as a phasecompensation film makes an angle of 167 to 173° with a horizontal line.

The lower alignment film is a vertical alignment film having a pre-tiltangle of 80 to 90° or less than 2°, and has an alignment angle of −40 to−50° with respect to a horizontal line. The upper alignment film is ahorizontal alignment film having a pre-tilt angle of less than 2° or 80to 90° in the opposite manner to the lower alignment film, and has analignment angle of 40 to 50° with respect to a horizontal line.

The phase compensation film has a phase compensation delay value of0.14-0.15 μm.

FIG. 8 is a graph showing a change in reflectance according to cell gap.As shown in FIG. 8, it can be found that the cell gap at whichreflectance reaches the maximum is about 5.6 μm.

FIGS. 9 and 11 show the axial arrangement of elements of the reflectiveHAN mode liquid crystal display according to the second embodiment ofthe present invention. In FIGS. 9 and 11, the reference numeral adesignates the rubbing axis of the lower substrate, b the rubbing axisof the upper substrate, c the optical axis of the phase compensationfilm, and d the transmission axis of the polarizer.

FIGS. 10 and 12 are graphs showing reflectance characteristics accordingto voltage in the reflective HAN mode liquid crystal displays of FIGS. 9and 11.

As shown in FIG. 9, the twist angle of liquid crystal molecules is 90°,the pre-tilt angle of the lower alignment film as a horizontal alignmentfilm is 0.2°, the pre-tilt angle of the upper alignment angle as avertical alignment film is 80 to 90°, the optical axis of the λ/4 filmmakes an angle of 167 to 173° with respect to a horizontal line, and thetransmission axis angle of the polarizer is 42 to 48°. Reflectancecharacteristics according to voltage in the liquid crystal display ofFIG. 9 are shown in FIG. 10.

As shown in FIG. 11, the twist angle of liquid crystal molecules is 90°,the pre-tilt angle of the lower alignment film as a vertical alignmentfilm is 0.2°, the pre-tilt angle of the upper alignment angle as ahorizontal alignment film is 80 to 90°, the optical axis of the λ/4 filmmakes an angle of 7 to 13° with respect to a horizontal line, and thetransmission axis angle of the polarizer is 42 to 48°. Reflectancecharacteristics according to voltage in the liquid crystal display ofFIG. 11 are shown in FIG. 12.

As described above, in the liquid crystal display according to thesecond embodiment of the present invention, the HAN mode liquid crystallayer is used while the pre-tilt angles of the lower and upper alignmentfilms are controlled in the opposite manner to each other. In addition,the optical axis angle of the λ/4 film and the transmission axis angleof the polarizer are suitably controlled. In this way, an optimal cellcan be embodied. Particularly, cell gap in the liquid crystal displayaccording to the second embodiment can be designed to have a largervalue than that of the first embodiment, so that it allows a moreincreased process margin to be ensured.

Embodiment 3

FIG. 13 is a cross-sectional view showing a multi-domain twisted HANmode liquid crystal display according to a third embodiment of thepresent invention.

As shown in FIG. 13, the liquid crystal display according to the thirdembodiment comprises a lower substrate 30 having a reflective electrode31, an upper substrate 33 having a color filter 34 and disposed oppositeto the lower substrate 30. A liquid crystal layer 37 is sandwichedbetween the lower and upper substrates. A lower alignment film 32 isformed on the reflective electrode 31. A unit pixel of the liquidcrystal display according to this embodiment is divided into first andsecond regions. A portion of the lower alignment film 32 correspondingto the first region is treated such that it vertically or horizontallyaligns liquid crystal molecules in the first region, and a portion ofthe lower alignment corresponding to the second region horizontally orvertically aligns liquid crystal molecules in the second region in theopposite manner to the first region. Moreover, an upper alignment film35 is formed on the color filter 33. The upper alignment film 35 istreated in the opposite manner to the lower alignment film 32. Namely, aportion of the upper alignment film 35 corresponding to the first regionis treated such that it vertically or horizontally aligns liquid crystalmolecules in the first region in the opposite manner to the portion ofthe lower alignment film corresponding to the first region. And aportion of the upper alignment film corresponding to the second regionis treated such that it horizontally or vertically aligns liquid crystalmolecules in the second region in the opposite manner to the firstregion.

The alignment angles of the lower alignment film 32 and the upperalignment film 36 are −40 to −50° and 40 to 50° with respect to ahorizontal line, respectively. Moreover, the lower alignment film 32 andthe upper alignment film 36 are treated to have different pre-tiltangles of less than 2° and 80 to 90°.

The liquid crystal layer 37 is formed of a twisted HAN mode liquidcrystal layer, which serves to converts the phase of transmitted lightdepending on whether electric field is applied or not. The twisted HANmode liquid crystal layer 37 is made of twisted nematic liquid crystalmolecules having a twist angle of 90°, and has a phase delay value (dΔn)of 0.36-0.40 μm.

Although not shown in FIG. 13, on the outer surface of the uppersubstrate 33, there is adhered a λ/4 film as a phase compensation filmserving to convert linearly polarized light into circularly polarizedlight. A polarizer for converting natural light from the external intolinearly polarized light is adhered on the λ/4 film. In this case, theλ/4 film has a phase compensation delay value of 0.14-0.15 μm, and itsoptical axis makes an angle of 167 to 173° with respect to a horizontalline. The transmission axis of the polarizer makes an angle of 42 to48°.

Embodiment 4

FIG. 14 is a cross-sectional view showing a transflective liquid crystaldisplay having a combined structure of twisted HAN mode and TN modeaccording to a fourth embodiment of the present invention.

As shown in FIG. 14, a unit pixel of a liquid crystal display accordingto this embodiment is divided into a reflection region and atransmission region. A reflective electrode 41 is formed on a portion ofthe lower substrate 40, which corresponds to the reflection region. Anupper substrate 43 having a color substrate 44 is disposed opposite tothe lower substrate 40.

On the lower substrate 40 including the reflective electrode 41, a loweralignment film 42 is formed. A portion of the lower alignment film 42corresponding to the reflection region is treated such that itvertically or horizontally aligns liquid crystal molecules in thereflection region. A portion of the lower alignment film 42corresponding to the transmission region is treated such that ithorizontally or vertically aligns liquid crystal molecules in thetransmission region in the opposite manner to the reflection region.Furthermore, an upper alignment film 45 is formed on the color filter44. A portion of the upper alignment film 45 corresponding to thereflection region is treated such that it vertically or horizontallyaligns liquid crystal molecules in the reflection region in the oppositemanner to the portion of the lower alignment film 42 corresponding tothe reflection region. A portion of the upper alignment film 45corresponding to the transmission region is treated such that itvertically or horizontally aligns liquid crystal molecules in thetransmission region in the opposite manner to the reflection region. Thealignment angles of the lower alignment film 42 and the upper alignmentfilm 45 are −40 to −50° and 40 to 50° with respect to a horizontal line,respectively. Moreover, the lower alignment film 42 and the upperalignment film 45 have different pre-tilt angles of less than 2° and 80to 90°.

Liquid crystal layers 47 and 48 are sandwiched between the lowersubstrate 40 and the upper substrate 43. The liquid crystal layers 47and 48 are made of two liquid crystal modes, i.e., a twisted HAN modeliquid crystal layer 47 and a TN mode liquid crystal layer 48. Thetwisted HAN mode liquid crystal layer 47 is located at the reflectionregion, and the TN mode liquid crystal layer 48 is located at thetransmission region. The HAN mode liquid crystal layer 47 and the TNmode liquid crystal layer 48 are made of twisted nematic liquid crystalshaving a twist angle of 90°, and have a phase delay value (dΔn) of0.36-0.40 μm.

As not shown in FIG. 14, on the outer surface of the upper substrate 43,there is adhered a λ/4 film as a phase compensation film serving toconvert linearly polarized light into circularly polarized light. Apolarizer is adhered on the λ/4 film. In this case, the λ/4 film has aphase compensation delay value of 0.14-0.15 μm, and its optical axismakes an angle of 167 to 173° with respect to a horizontal line. Thetransmission axis of the polarizer makes an angle of 42 to 48°.

Meanwhile, as shown in FIG. 14, when the thickness of the liquid crystallayer in the reflection region is d, cell gap in the transmission regionis designed as d×2, such that the reflection region and the transmissionregion have the same phase delay value (dΔn).

Furthermore, when liquid crystal molecules having the same refractiveindex (Δn) are used, the transmission region having a TN mode isdesigned such that the distance between the reflective electrode and acounter electrode is about 4.8 μm, and the reflection region having thetwisted HAN mode is designed such that the reflective electrode and thecounter electrode is about 5.6 μm. In this case, the absorption axis ofthe polarizer is designed in the same manner. In addition, concave orconvex step height can be given in the upper or lower substrate tocontrol cell gap.

As described above, according to the present invention, the design of aliquid crystal cell is optimized so that reflectance according tovoltage can be increased. For this reason, contrast ratio and colorcharacteristics can be increased, thereby providing a liquid crystaldisplay having good display characteristics.

In addition, the liquid crystal display of the present invention can bedesigned to have increased cell gap as compared to that of the priorliquid crystal display. This allows process margin to be increased andthus production yield to be improved.

Although a preferred embodiment of the present invention has beendescribed for illustrative purposes, those skilled in the art willappreciate that various modifications, additions and substitutions arepossible, without departing from the scope and spirit of the inventionas disclosed in the accompanying claims.

1. A liquid crystal display which comprises: a lower substrate having areflective electrode; a lower alignment film formed on the lowersubstrate; an upper substrate having a color filter and disposedopposite to the lower substrate; an upper alignment film formed on theupper substrate; a liquid crystal layer sandwiched between the lower andupper substrates; a phase compensation film adhered on the outer surfaceof the upper substrate and serving to convert linearly polarized lightinto circularly polarized light; and a polarizer adhered on the phasecompensation film and serving to convert natural light from the externalinto linearly polarized light, wherein a unit pixel of the liquidcrystal display is divided into first and second regions; a portion ofthe lower alignment film corresponding to the first region is treatedsuch that it vertically or horizontally aligns liquid crystal moleculesin the first region, and a portion of the lower alignment regioncorresponding to the second region is treated such that it horizontallyor vertically aligns liquid crystal molecules in the second region inthe opposite manner to the first region, the lower alignment film havingan alignment angle of −40 to −50° with respect to a horizontal line; aportion of the upper alignment film corresponding to the first region istreated such that it vertically or horizontally aligns liquid crystalmolecules in the first region in the opposite manner to the portion ofthe lower alignment film corresponding to the first region, and aportion of the upper alignment region corresponding to the second regionis treated such that it horizontally or vertically aligns liquid crystalmolecules in the second region in the opposite manner to the firstregion, the upper alignment film having an alignment angle of 40 to 50°with respect to a horizontal line; the liquid crystal layer is aHAN-mode liquid crystal layer serving to convert the phase oftransmitted light depending on electric field application or no electricfield application and has a phase delay value (dΔn) of 0.36-0.40 μm; thephase compensation film has a phase compensation function of λ/4 andalso has an optical axis making an angle of 167 to 173° with ahorizontal line; and the polarizer has a transmission axis making anangle of 42 to 48° with a horizontal line.
 2. The liquid crystal displayof claim 1, wherein the lower alignment film and the upper alignmentfilm have different pre-tilt angles of 2° or less and 80 to 90°.
 3. Theliquid crystal display of claim 2, wherein the liquid crystal layer ismade of twisted nematic liquid crystals having a twist angle of 90°. 4.The liquid crystal display of claim 1, wherein the phase compensationfilm has a phase compensation delay value of 0.14-0.15 μm.